1. Field of the Invention
The present invention relate to a two-input/one-output DC-DC converter and a power supplying system which is equipped with the same and which supplies power to a load from two DC power sources, and more particularly, a power supplying system for parallel feeding to a load from a fuel cell and from a secondary battery.
2. Description of the Related Art
Conventionally, a power source system for parallel feeding to a load is known which uses an independent power source, such as a solar cell or a fuel cell, and a secondary battery (see, for example, JP 2003-134691A). According to this power source system, three boost converters are provided between the fuel cell and the secondary battery, power is supplied to the load from the fuel cell through a first boost converter, and excess power of the fuel cell is stored in the secondary battery through the second boost converter. Moreover, power is supplied to the load from the secondary battery through the third boost converter to make up the shortage of supplied power from the fuel cell.
Regarding fuel cell electric vehicles, there is known a power supplying system which adjusts a voltage through a DC-DC converter capable of both boosting and bucking and which connects a low-voltage system of 12 V including vehicle accessories and a low-voltage battery, and, a high-voltage system of 350 V including fuel-cell accessories and a load together (see, for example, Japan Patent No. 3744456). According to this power supplying system, a connection is established in such a way that an output voltage by the fuel cell is directly applied to the load.
In the technical field of fuel cells, there is a technological challenge for reducing the number of cells in a stack of a fuel cell from the standpoint of cost reduction and of a spatial restriction. When the number of cells in the stack is reduced, the output voltage by the fuel cell is decreased, so that if, like Japan Patent No. 3744456, an output voltage by a fuel cell is directly applied to a load, the load cannot acquire necessary motor output power. Conventionally, there is also known a system which further has a heavy-electricity booster at the output side of a fuel cell in addition to the configuration of the power supplying system disclosed in Japan Patent No. 3744456, and which supplies power to a load through the high power booster (see, for example, Japan Patent No. 3709789). This system also has a secondary battery capable of being charged by the high power booster, and the secondary battery can supply power to the fuel-cell accessories and to the load.
According to the systems disclosed in JP2003-134691A and in Japan Patent No. 3744456, however, when a fluctuation range of a voltage of a fuel cell is large or when a battery voltage reduction occurs, it is difficult to stably supply power to individual system elements. Moreover, according to the system disclosed in Japan Patent No. 3709789, when the voltage of the fuel cell or the voltage of a battery (the secondary battery or the low-voltage battery) fluctuates, it becomes difficult to supply power to the fuel cell accessories and to the load at a stable voltage.